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Abstract:

A method for manufacturing a galvanized steel sheet, includes:
galvanizing a steel sheet; bringing the surface of the steel sheet into
contact with an aqueous solution containing zinc ion in the range of 5 to
100 g/l as the zinc ion concentration, having a pH of 4 to 6, and having
a liquid temperature of 20 to 70° C., holding the steel sheet for
1 to 60 seconds; and then washing and drying the steel sheet. The
solution containing zinc is preferably one containing zinc sulfate, for
example. According to the method, a galvanized steel sheet having an
oxide layer having an average thickness of 10 nm or more and mainly
containing zinc formed on the surface of the steel sheet and having
excellent press formability can be stably manufactured in a short time.

Claims:

1. A method for manufacturing a galvanized steel sheet, comprising:
galvanizing a steel sheet; bringing the steel sheet into contact with an
aqueous solution; holding the steel sheet for 1 to 60 seconds after the
termination of the contact treatment; and washing with water and drying
the steel sheet to thereby form an oxide layer on the surface of the
steel sheet, the aqueous solution for use in the contact treatment of the
steel sheet containing zinc ion in the range of 5 to 100 g/l as the zinc
ion concentration, having a pH of 4 to 6, and having a liquid temperature
of 20 to 70.degree. C.

2. The method for manufacturing a galvanized steel sheet according to
claim 1, wherein the aqueous solution contains zinc sulfate.

3. The method for manufacturing a galvanized steel sheet according to
claim 1, wherein a liquid film to be formed on the surface of the steel
sheet after the steel sheet contacts the aqueous solution is 5 to 30
g/m.sup.2.

4. A galvanized steel sheet, which is manufactured according to the
method for manufacturing a galvanized steel sheet according to claim 1,
an oxide layer mainly containing zinc as a metal component being formed
on the surface of the steel sheet in such a manner as to have an average
thickness of 10 nm or more.

5. The method for manufacturing a galvanized steel sheet according to
claim 2, wherein a liquid film to be formed on the surface of the steel
sheet after the steel sheet contacts the aqueous solution is 5 to 30
g/m.sup.2.

6. A galvanized steel sheet, which is manufactured according to the
method for manufacturing a galvanized steel sheet according to claim 2,
an oxide layer mainly containing zinc as a metal component being formed
on the surface of the steel sheet in such a manner as to have an average
thickness of 10 nm or more.

7. A galvanized steel sheet, which is manufactured according to the
method for manufacturing a galvanized steel sheet according to claim 3,
an oxide layer mainly containing zinc as a metal component being formed
on the surface of the steel sheet in such a manner as to have an average
thickness of 10 nm or more.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a method for stably manufacturing
a galvanized steel sheet having a low sliding resistance during press
forming and excellent press formability and a galvanized steel sheet
having excellent press formability.

BACKGROUND ART

[0002] The galvanized steel sheet has been widely utilized in wide ranging
fields focusing on the application to automobile bodies. A galvanized
steel sheet in such application is press formed for use. However, the
galvanized steel sheet has a disadvantage in that the press formability
is inferior to that of a cold-rolled steel sheet. This is because the
sliding resistance of the galvanized steel sheet in a press die is higher
than that of the cold-rolled steel sheet. More specifically, the
galvanized steel sheet becomes difficult to flow into a press die in a
portion where the sliding resistance between a die and a bead, which
easily causes fracture of the steel sheet.

[0003] Here, particularly a galvannealed steel sheet that has been
subjected to alloying treatment after hot dip galvanizing treatment among
galvanized steel sheets has more excellent weldability and coatability
than those of a hot-dip zinc-plated steel sheet that has not been
subjected to alloying treatment, and thus has been more preferably used
as automobile bodies.

[0004] The galvannealed steel sheet is one in which an Fe--Zn alloy phase
is formed by galvanizing a steel sheet, and heating the same so that Fe
in the steel sheet and Zn in a plating layer are dispersed to cause an
alloying reaction. The Fe--Zn alloy phase is a coating film generally
containing a Γ phase, a δ1 phase, and a ξ phase and
has a tendency that the hardness and the melting point decrease with a
reduction in the Fe concentration, i.e., in the order of Γ
phase→δ1 phase→ξ phase. Therefore, a coating
film having a high Fe concentration in which the hardness is high, the
melting point is high, and adhesion is difficult occur is effective from
the viewpoint of slidability. A galvannealed steel sheet in which the
press formability is emphasized is manufactured in such a manner that the
average Fe concentration in the coating film is slightly high.

[0005] However, the coating film having a high Fe concentration has
problems in that the Γ phase that is hard and brittle is easily
formed on the plated-steel sheet interface and a phenomenon of separation
from the interface during processing, i.e., a so-called powdering, is
likely to occur.

[0006] As methods for solving the problems, Patent Document 1 and Patent
Document 2 disclose a technique of increasing the weldability and the
processability by subjecting the surface of a galvanized steel sheet to
electrolysis treatment, immersion treatment, coating oxidation treatment,
or heat-treatment to form an oxide film mainly containing ZnO.

[0007] However, when the techniques of Patent Document 1 and Patent
Document 2 are applied to a galvannealed steel sheet, the surface
reactivity becomes poor due to the presence of an Al oxide and an effect
of improving the press formability cannot be stably obtained because the
surface irregularities are large. More specifically, since the surface
reactivity is low, it is difficult to form a given film on the surface
even when the electrolysis treatment, immersion treatment, coating
oxidation treatment, heat-treatment, or the like is performed and the
film thickness is small in a portion where the reactivity is low, i.e., a
portion in which the number of Al oxides is large. Since the surface
irregularities are large, the surface convex portions directly contact a
press die during press forming. The sliding resistance in contact
portions of thin portions of the convex portions and the die becomes
large, and thus an effect of improving the press formability is not
sufficiently obtained.

[0008] Patent Document 3 discloses a technique of forming an oxide layer
on a plated surface layer by hot dip galvanizing a steel sheet, alloying
the same by heat treatment, subjecting the resultant steel sheet to
temper rolling, bringing the same into contact with an acidic solution
having pH buffer action, holding the same for 1 to 30 seconds, and then
washing with water.

[0009] Similarly, as a method for uniformly forming an oxide layer on a
surface flat portion of a hot dip galvanized steel sheet that has not
been subjected to alloying treatment, Patent Document 4 discloses a
method including bringing a hot dip galvanized steel sheet after temper
rolling into contact with an acidic solution having pH buffer action,
holding the same for a given period of time in a state where a liquid
film of the acidic solution is formed on the surface of the steel sheet,
and then washing with water and drying the same.

[0014] When the techniques disclosed in Patent Document 3 and Patent
Document 4 are applied, favorable press formability can be obtained under
former manufacturing conditions. However, in recent years, the
development of a manufacturing method for generating a thicker oxide film
in a shorter period of time has been demanded in order to increase the
productivity. When performed under such conditions, a sufficient oxide
film is not formed and favorable press formability is not obtained in
some cases in the techniques disclosed in Patent Document 3 and Patent
Document 4.

[0015] In view of such circumstance, it is an object of the present
invention to provide a method capable of stably manufacturing a
galvanized steel sheet having excellent press formability even in a short
time and a galvanized steel sheet having excellent press formability.

DISCLOSURE OF INVENTION

[0016] The present inventors have repeatedly conducted extensive research
in order to solve the problems. As a result, the following findings have
been obtained.

[0017] The acidic solution for use in the techniques of Patent Document 3
and Patent Document 4 has pH buffer action in order to promote the
dissolution of zinc. Therefore, it is considered that an increase in the
pH is delayed, and thus the formation of an oxide layer is delayed. In
order to compensate zinc for forming an oxide layer with zinc eluting
from a plated coating film, an elution time of zinc is included in a
generation time of the oxide film. As a result, it is considered that
generating a thick oxide film in a short time becomes difficult.

[0018] Then, the present inventors have devised a technique of generating
an oxide film in a shorter time by omitting an elution time of zinc by
blending zinc ion in an aqueous solution for generating an oxide film
beforehand. However, the formation of an oxide film has not been promoted
merely by blending zinc ion in an aqueous solution beforehand.
Particularly in the case where the pH is 2 described in Examples of
Patent Document 3 and Patent Document 4, even when zinc is blended in a
treatment liquid, the formation of an oxide film has not been promoted.

[0019] This is considered to be because, according to the techniques of
Patent Document 3 and Patent Document 4, an environment is established in
which a zinc oxide is likely to generate because the pH near the surface
increases due to the reduction of hydrogen ion occurring simultaneously
with the elution of zinc, but the pH near the surface does not increase
merely by blending zinc ion in an aqueous solution, and thus an
environment is not established in which a zinc oxide is likely to
generate.

[0020] Then, the present inventors have devised a technique of setting the
pH of an aqueous solution to 4 to 6, the pH at which a zinc oxide is
likely to generate. Then, the present inventors have found that, by
setting the pH of a treatment liquid to 4 to 6, zinc is generated as a
hydroxide due to a slight increase in the surface pH caused by slight
elution of zinc of a plated coating film.

[0021] The present invention has been accomplished based on the findings,
and the gist is as follows.

[0022] [1] A method for manufacturing a galvanized steel sheet, includes
galvanizing a steel sheet, bringing the steel sheet into contact with an
aqueous solution, holding the steel sheet for 1 to 60 seconds after the
termination of the contact treatment, and then washing with water and
drying the steel sheet to thereby form an oxide layer on the surface of
the steel sheet, in which the aqueous solution for use in the contact
treatment of the steel sheet contains zinc ion in the range of 5 to 100
g/l as the zinc ion concentration, has a pH of 4 to 6, and has a liquid
temperature of 20 to 70° C.

[0023] [2] The method for manufacturing a galvanized steel sheet according
to [1] above, in which the aqueous solution contains zinc sulfate.

[0024] [3] The method for manufacturing a galvanized steel sheet according
to [1] or [2] above, in which a liquid film to be formed on the surface
of the steel sheet after the steel sheet contacts the aqueous solution is
5 to 30 g/m2.

[0025] [4] A galvanized steel sheet, which is manufactured according to
the method for manufacturing a galvanized steel sheet according to any
one of [1] to [3] above, in which an oxide layer mainly containing zinc
as a metal component is formed on the surface of the steel sheet in such
a manner as to have an average thickness of 10 nm or more.

[0026] In the invention, the galvanized steel sheet is a plated steel
sheet having a coating film containing zinc as the main component formed
on the surface and includes a hot dip galvanized steel sheet (abbreviated
as a GI steel sheet), a galvannealed Steel Sheet (abbreviated as a GA
steel sheet), an electrogalvanized steel sheet (abbreviated as an EG
steel sheet), a vapor deposition galvanized steel sheet, an alloy
galvanized steel sheet containing an alloy element of Fe, Al, Ni, MgCo,
or the like, etc.

BRIEF DESCRIPTION OF DRAWINGS

[0027] FIG. 1 is a view of a principal part of an oxide layer formation
treatment facility used in Examples.

[0029] FIG. 3 is a schematic perspective view showing the shape and the
size of a bead in FIG. 2.

[0030] FIG. 4 is a schematic perspective view showing the shape and the
size of the bead in FIG. 2.

[0031] FIG. 5 is a view showing influence of the zinc ion concentration on
the oxide film thickness.

BEST MODES FOR CARRYING OUT THE INVENTION

[0032] In the invention, when forming an oxide layer on the surface of a
steel sheet by galvanizing a steel sheet, bringing the steel sheet into
contact with an aqueous solution, holding the steel sheet for 1 to 60
seconds after the termination of the contact treatment, and then washing
with water and drying the steel sheet, the aqueous solution contains zinc
ion in the range of 5 to 100 g/l as the zinc ion concentration, the pH is
4 to 6, and the liquid temperature is 20 to 70° C. To prepare an
aqueous solution containing zinc ion in a given concentration and having
a specified pH and a specified liquid temperature as described above as
the aqueous solution for use in the contact treatment of the steel sheet
is an important requirement and a feature in the invention. Thus, an
oxide layer sufficient for securing favorable press formability can be
formed in a short time.

[0033] The "after the termination of the contact treatment" refers to
"after the termination of an immersion process" in the case of immersion
treatment, "after the termination of a spraying process" in the case of
spraying treatment, and "after the termination of a coating process" in
the case of roll coating.

[0034] The use of an aqueous solution containing zinc ion as the aqueous
solution for use in the contact treatment of the steel sheet allows
omission of an elution time of zinc. In this case, the zinc ion is in the
range of 5 to 100 g/l as the zinc ion concentration. When the zinc ion
concentration is lower than 5 g/l, sufficient zinc is not supplied,
resulting in a failure of the formation of an oxide layer. In contrast,
when the zinc ion concentration exceeds 100 g/l, the concentration of
sulfuric acid contained in the oxide layer to be formed becomes high,
resulting in concern about contamination of a treatment liquid when the
oxide dissolves in chemical conversion treatment to be carried out
thereafter.

[0035] In order to form a stable zinc compound as an oxide layer, it is
preferable to add zinc ion as a sulfate. It is considered that when zinc
ion is added as a sulfate, sulfuric acid ion is taken into an oxide layer
to be formed to thereby produce an effect of stabilizing the oxide layer.

[0036] As described above, the formation of an oxide film is not promoted
merely by blending zinc ion in a treatment liquid beforehand. Then, in
the invention, the pH needs to be set to 4 to 6, at which a zinc oxide
easily generates. When the pH of a treatment liquid is set to 4 to 6,
zinc generates as a hydroxide due to a slight increase in the surface pH
caused by slight elution of zinc of a plated coating film. As a result
thereof, the zinc elution time can be omitted and the generation of a
zinc oxide can be achieved. When the pH exceeds 6, zinc ion precipitates
in the aqueous solution (formation of a hydroxide) and is not formed as
an oxide on the surface of the steel sheet. When the pH is lower than 4,
the formation of the oxide layer is hindered due to the delay of an
increase in the pH as described above.

[0037] The temperature of the aqueous solution is 20 to 70° C.
Since the oxide layer formation reaction occurs when holding the steel
sheet in a given period of time after contacting the aqueous solution, it
is effective to control the sheet temperature during holding in the range
of 20 to 70° C. When the sheet temperature is lower than
20° C., a long period of time is required for the oxide layer
generation reaction, resulting in a reduction in the productivity. In
contrast, when the sheet temperature exceeds 70° C., a reaction
relatively quickly proceeds but treatment unevenness is likely to occur
on the surface of the steel sheet.

[0038] The aqueous solution used in Patent Document 3 and Patent Document
4 has a feature in that the aqueous solution is acidic and has pH buffer
action. In the invention, however, since an aqueous solution containing
zinc ion is used, a sufficient oxide layer can be formed even when the
dissolution of zinc is not caused by increasing the pH of the aqueous
solution. A prompt increase in the pH is considered to be advantageous
for the formation of an oxide. Therefore, the pH buffer action is not
necessarily indispensable.

[0039] In the invention, an oxide layer excellent in slidability can be
stably formed when zinc is contained in the aqueous solution contacting
the surface of the steel sheet. Therefore, even when other metal ions,
inorganic compounds, and the like are contained as impurities or
intentionally contained in the aqueous solution, the effects of the
invention are not impaired. Even when N, P, B, Cl, Na, Mn, Ca, Mg, Ba,
Sr, Si, and the like are taken into the oxide layer, it can be applied
insofar as the effects of the invention are not impaired.

[0040] Preferably, after bringing a galvanized steel sheet into contact
with the aqueous solution containing the above, the aqueous solution is
present on the surface of the steel sheet in the form of a thin liquid
film. This is because when the amount of the aqueous solution present on
the surface of the steel sheet is large, the pH of the aqueous solution
is hard to increase even when the dissolution of zinc occurs, and a long
period of time is required for the formation of the oxide layer. From
this viewpoint, it is preferable and effective to adjust the amount of an
aqueous solution film to be formed on the surface of the steel sheet to
30 g/m2 or lower. In order to prevent the liquid film from drying,
the amount of the liquid film of 5 g/m2 or more is suitable. As
described above, the liquid film to be formed on the surface of the steel
sheet after contacting the aqueous solution is preferably 5 to 30
g/m2. The adjustment of the amount of the aqueous solution film can
be performed by a squeeze roll, air wiping, or the like.

[0041] The time (retention time before washing with water) before washing
with water after immersion in the aqueous solution is 1 to 60 seconds.
When the time before washing with water is lower than 1 second, the
aqueous solution is washed away before a sufficient oxide layer is
formed, and thus an effect of improving the slidability is not obtained.
In contrast, when the time before washing with water exceeds 60 seconds,
the productivity decreases. Since the object of the invention is to
stably manufacture a galvanized steel sheet even in a short time, the
retention time is 60 seconds or lower for sufficiently demonstrating the
effects of the invention.

[0042] As described above, on the surface of the plated steel sheet of the
invention, an oxide layer mainly containing zinc as a metal component and
having an average thickness of 10 nm or more is obtained.

[0043] The "mainly containing zinc" refers to containing zinc in a
proportion of 50% by mass or more as a metal component.

[0044] The oxide layer in the invention refers to a layer containing an
oxide and/or a hydroxide mainly containing zinc as a metal component. The
average thickness of the oxide layer is required to be 10 nm or more.
When the average thickness of the oxide layer is small, e.g., lower than
10 nm, an effect of reducing sliding resistance becomes insufficient. In
contrast, when the average thickness of the oxide layer containing zinc
as an essential ingredient exceeds 100 nm, there is a tendency that the
coating film breaks during press processing, the sliding resistance
increases, and the weldability decreases. Thus, such a thickness is not
preferable.

[0045] Methods for bringing the galvanized steel sheet into contact with
the aqueous solution containing zinc are not particularly limited. For
example, a method for immersing the plated steel sheet in the aqueous
solution, a method for spraying the aqueous solution to the plated steel
sheet, a method for applying the aqueous solution to the plated steel
sheet with a coating roll, and the like are mentioned. It is preferable
for the aqueous solution to be finally present on the surface of the
steel sheet in the form of a thin liquid film.

[0046] For manufacturing the galvannealed steel sheet according to the
invention, Al needs to be added into a plating bath but additional
element ingredients other than Al are not particularly limited. More
specifically, even when Pb, Sb, Si, Sn, Mg, Mn, Ni, Ti, Li, Cu, and the
like other than Al are contained or added, the plating bath can be
applied insofar as the effects of the invention are not impaired.

EXAMPLES

[0047] Next, the invention will be described in more detail with reference
to Examples.

[0048] A GI steel sheet was produced by performing hot dip galvanizing in
which the deposit amount per surface was 45 g/m2 and the Al
concentration was 0.20% by mass on a cold-rolled steel sheet having a
sheet thickness of 0.8 mm, and then performing temper rolling. A GA steel
sheet was obtained by forming a plated coating film in which the deposit
amount per surface was 45 g/m2, the Fe concentration was 10% by
mass, and the Al concentration was 0.20% by mass on a cold-rolled steel
sheet having a sheet thickness of 0.8 mm by a standard galvannealing
method, and further performing temper rolling. An EG steel sheet was
produced by having a plated coating film having a deposit amount per
surface of 30 g/m2 on a cold-rolled steel sheet having a sheet
thickness of 0.8 mm by a standard electrogalvanizing method.

[0049] Subsequently, an oxide layer was formed using a treatment facility
having a structure shown in FIG. 1. First, steel sheets S, such as the GI
steel sheet, the GA steel sheet, and the EG steel sheet obtained above
were immersed in aqueous solutions in which the treatment liquid
composition, the temperature, and the pH were different from each other
as shown in Tables 1-1 and 1-2 in a solution bath 2. Subsequently, the
amount of liquid films on the surface of the steel sheets was adjusted
with a squeeze roll 3. The adjustment of the amount of liquid films was
performed by changing the pressure of the squeeze roll. Subsequently, the
steel sheets were made to pass through a washing bath 5 and a washing
bath 6 without being treated, hot water of 50° C. was sprayed to
the steel sheets in a washing bath 7 for washing, and the steel sheets
were dried with a drier 8, so that an oxide layer is formed on the plated
surface. A washing bath 1 can be provided before the solution bath 2.

[0050] As the aqueous solution for use in the immersion treatment in the
solution bath 2, an aqueous solution was used to which a given amount of
zinc sulfate heptahydrate was added in order to add zinc ion. For
comparison, a solution containing 20 g/L of sodium acetate whose pH was
adjusted with sulfuric acid was also used in some cases.

[0051] The retention time before washing with water was the time before
washing in the washing bath 7 was started after adjusting the amount of
liquid films with the squeeze roll 3 and was adjusted by changing the
line speed. Some of the steel sheets were produced by washing immediately
after squeezing using a shower washing device 4 at the exit side of the
squeeze roll 3.

[0052] Next, the steel sheets produced as described above were judged
whether or not they have an appearance sufficient as an exterior panel
for automobiles, and also the measurement of a friction coefficient as a
method for simply evaluating the press formability and a spherical head
bulging test was carried out in order to simulate the actual formability
in detail were carried out. The measurement methods are as follows.

[0053] In order to evaluate the press formability, the friction
coefficient of each test piece was measured as follows.

[0054] FIG. 2 is a schematic front view showing a friction coefficient
measuring device. As shown in FIG. 2, a friction coefficient measuring
sample 11 extracted from the test piece is fixed to a sample stand 12.
The sample stand 12 is fixed to the upper surface of a horizontally
movable slide table 13. On the lower surface of the slide table 13, a
vertically movable slide table support stand 15 having a roller 14
contacting the lower surface of the slide table 13. By pressing up the
same, a first load cell 17 for measuring a pressing load N to the
friction coefficient measuring sample 11 by a bead 16 is attached to the
slide table support stand 15. In order to measure a sliding resistance F
for horizontally moving the slide table 13 along a rail 19 in the state
where the pressing force was made to act, a second load cell 18 is
attached to one end of the slide table 13. As a lubricant, a cleaning oil
for pressing, Preton R352L manufactured by Sugimura Chemical Industrial
Co., Ltd., was applied onto the surface of the friction coefficient
measuring sample 11, and thus a test was carried out.

[0055] FIGS. 3 and 4 are schematic perspective view showing the shape and
the size of the used bead. The bead 16 slides while the lower surface of
the bead 16 being pressed against the surface of the sample 11. In the
bead 16 shown in FIG. 3, the width is 10 mm, the length in the sliding
direction of the sample is 12 mm, and each end in the sliding direction
of the lower surface of the bead 16 is curved with a curvature of 4.5
mmR. The lower surface of the bead 16 against which the sample is pressed
has a plane with a width of 10 mm and a length in the sliding direction
of 3 mm. In the bead 16 shown in FIG. 4, the width is 10 mm, the length
in the sliding direction of the sample is 69 mm, and each end in the
sliding direction of the lower surface of the bead 16 is curved with a
curvature of 4.5 mmR. The lower surface of the bead 16 against which the
sample is pressed has a plane with a width of 10 mm and a length in the
sliding direction of 60 mm.

[0056] The friction coefficient measurement test was carried out under two
conditions shown below.

[Condition 1]

[0057] The bead shown in FIG. 3 was used, the pressing load N was 400 kgf,
and the sample drawing rate (horizontal movement rate of the slide table
13) was 100 cm/min.

[Condition 2]

[0058] The bead shown in FIG. 4 was used, the pressing load N was 400 kgf,
and the sample drawing rate (horizontal movement rate of the slide table
13) was 20 cm/min.

[0059] The friction coefficient between the test piece and the bead was
calculated based on Equation: μ=F/N.

(2) Spherical Head Bulging Test

[0060] A test piece having a size of 200×200 mm was subjected to
bulge forming using a 150 mmφ punch by a liquid pressure bulge
testing machine. Then, the maximum forming height when the test piece was
broken was measured. During the test, a wrinkle pressing force of 100 Ton
was applied in order to prevent inflow of materials, and a lubricating
oil was applied only to the surface which the punch contacted. The used
lubricating oil is the same as that of the friction coefficient
measurement test described above.

(3) Measurement of Thickness of Oxide Layer (Oxide Film Thickness)

[0061] An Si wafer on which a thermal oxidation SiO2 film having a
film thickness of 96 nm was formed was used as a reference substance, and
the average thickness of the oxide layer in terms of SiO2 was
determined by measuring the OKαX rays by an x-ray fluorescence
spectrometer. The analysis area is 30 mmφ.

[0062] The test results obtained in the above are shown in Tables 1-1 and
1-2.

The following items were clarified from the test results shown in Tables
1-1 and 1-2.

[0063] (1) Since Nos. 1, 47, and 60 were not treated with a solution, an
oxide film sufficient for increasing the slidability was not formed on
the flat portion. Thus, the friction coefficient is high.

[0064] (2) Nos. 2 to 4, Nos. 48 to 50, and Nos. 61 to 63 are comparative
examples using an acidic solution having pH buffer action. In the case of
the treatment of 30 seconds or more, the friction coefficient is low and
the maximum forming height is large but in the case of the treatment of
10 seconds, a sufficient reduction in the friction coefficient and an
increase in the maximum forming height are not satisfied.

[0065] (3) Nos. 5 to 7 are comparative examples using an acidic solution
having pH buffer action. High friction coefficients are exhibited.

[0066] (4) Nos. 8 to 10, Nos. 51 to 53, and Nos. 64 to 66 are comparative
examples in which zinc ion is contained but the amount is smaller than
the range of the invention. In the case of the treatment of 30 seconds or
more, the friction coefficient is low and the maximum forming height is
large but in the case of the treatment of 10 seconds, a sufficient
reduction in the friction coefficient and an increase in the maximum
forming height are not satisfied.

[0067] (5) Nos. 11 to 13, Nos. 54 to 56, and Nos. 67 to 69 are examples of
the invention that were treated with a solution containing zinc ion, in
which the friction coefficient decreases and also the maximum forming
height increases. Nos. 14 to 16 and Nos. 44 to 46 are examples of the
invention in which the treatment conditions were the same as those of
Nos. 11 to 13 and the zinc ion concentration in the liquid was increased.
The friction coefficient becomes stable at lower levels and also the
maximum forming height further increases. Similarly, Nos. 57 to 59 and
Nos. 70 to 72 are examples of the invention in which the treatment
conditions were the same as those of Nos. 54 to 56 and the zinc ion
concentration in the liquid was increased. The friction coefficient
becomes stable at lower levels and also the maximum forming height
further increases.

[0068] (6) Nos. 17 to 22 are examples in which a solution film was formed
on the surface of the steel sheets and the time until washing with water
was carried out was changed. In No. 17 which was washed with water
without being held, the friction coefficient merely slightly decreases.
In contrast, in Nos. 18 to 22 in which the retention time was 1 second or
more, the friction coefficient decreases and also the bulging properties
stably increase.

[0069] (7) Nos. 23 to 40 are examples in which the treatment liquid
temperature was changed. In Nos. 23 to 25 having a low treatment liquid
temperature, effects of improving the friction coefficient and the
maximum forming height are not sufficient as compared with the other
examples. In contrast, Nos. 32 to 34 are examples having a high treatment
liquid temperature and effects of improving the friction coefficient or
the maximum forming height were sufficient but treatment unevenness was
observed in many portions and thus the appearance was not favorable as an
exterior panel for automobiles.

[0070] (8) Nos. 35 to 40 are examples of the invention in which the liquid
film formation amount was changed relative to Nos. 20 to 22. A comparison
between the samples in which the retention time until washing with water
was carried out is the same shows that when the liquid film amount was
large, a sufficient reduction in the friction coefficient and an increase
in the maximum forming height are achieved but the friction coefficient
was slightly high and also the maximum forming height was small as
compared with the samples in which the liquid film amount was small.

[0071] (9) Nos. 41 to 43 are comparative examples using a treatment liquid
in which pH is lower than the range of the invention. The effect of
reducing the friction coefficient is not observed and also an increase in
the maximum forming height is not observed as compared with Nos. 20 to
22.

[0072] FIG. 5 is a view showing the influence of the zinc ion
concentration on the oxide film thickness using Nos. 8 to 22 and Nos. 44
to 46 of Tables 1-1 and 1-2. FIG. 5 shows that the oxide film has a
sufficient thickness even when the retention time is short (e.g., 10
seconds) in the case where the zinc concentration is 5 g/l or more, and
the problem of the invention that the oxide film thickness becomes small
when the retention time is short is solved.

INDUSTRIAL APPLICABILITY

[0073] According to the present invention, a galvanized steel sheet having
a low sliding resistance during press forming and excellent press
formability can be stably manufactured at a saved space even under
short-time manufacturing conditions. For example, even when a high
strength galvanized steel sheet which has a high forming load and is
likely to cause die galling, the sliding resistance during press forming
is low and excellent press formability can be achieved. Since the press
formability is excellent, the invention can be applied to wide ranging
fields focusing on the application to automobile bodies.